Prior art linear-beam microwave tubes in UHF television transmitters operating in the frequency range from 450 MHz to 900 MHz have suffered from spurious signals and oscillation caused by secondary electrons back-streaming from the collector of the tube and returning through the tube in a direction reverse to that of the primary beam of electrons. These secondary electrons have been sufficiently numerous to cause (1) an increase in the observed noise level at the output of the tube, (2) ringing on heavy pulse signals and (3) oscillation. The secondary electrons are formed by impact of the high energy electron beam in the collector region. High energy secondary electrons having approximately the same emission velocity as the primary or beam electrons result from so-called "elastic" collisions. While these high energy secondary electrons are particularly troublesome, low energy secondary electrons are also emitted and can disturb normal tube operation. The secondary electrons can follow the beam path in a direction reverse to that of the beam and reach the input end of the tube where they are subjected to the high overall gain of the tube resulting in the aforementioned oscillation or ringing.